Time delay and integration (TDI) is an imaging technique that uses an area array image sensor to capture images from an imaging platform that is moving relative to the imaged object or scene. As the object or scene moves across the array, the image sensor takes multiple samples and sums these samples in order to improve the signal to noise ratio as compared to a single line capture of the image sensor. This improvement to signal to noise ratio makes TDI imaging techniques particularly well-suited to applications with low light levels or fast moving objects. Example applications can include medical imaging, machine vision, roll or conveyor belt inspection systems or terrestrial imaging from aircraft or satellites.
Conventionally, charge-coupled device (CCD) technology has been used for TDI applications because CCDs intrinsically operate by shifting charge from pixel to pixel across the image sensor. This shifting of charge allows the CCD image sensor to accomplish the integration (or adding) of the multiple samples without complex circuitry to perform the integration operation and the accompanying noise. However, CCD technology is relatively expensive to fabricate and CCD imaging devices consume much more power than comparably sized devices implemented using complementary metal-oxide semiconductor (CMOS) technology.
Implementing a TDI sensor using CMOS technology not only allows for a lower power designs but also allows for the integration of other electronics with the TDI image sensor. A CMOS TDI implementation requires additional circuitry to perform the addition or integration that is performed by shifting charges in a CCD. The signal is converted to voltage directly inside the CMOS pixels and requires adder circuitry outside the pixel array. Typically, this is performed in the digital domain after analog-to-digital conversion using a memory element and adder circuits.
Image sensors, including CMOS TDI imaging devices, can produce an undesirable response known as fixed pattern noise (FPN). FPN produces a non-image pattern that is caused by variance in the pixel response and non-uniformity of the circuitry used to read the pixel response. This non-uniformity can be the result of manufacturing constraints and environmental conditions that cause the pixels to have different responses despite receiving substantially similar levels of light.
One approach for correcting fixed pattern noise errors employs factory calibration of the detector array. Factory calibration involves exposing the array to a uniform source and tabulating the response of each detector in the array. The tabulated entries consist of gain and offset corrections for each detector in the array. The entries in the table can be applied against corresponding detectors to generate a corrected image. The factory calibration solution, however, suffers from multiple drawbacks. First, the pixel offset errors may not be linearly dependent, rather they may have non-linear temperature variations. Thus, factory calibration must take place over a broad range of temperatures to perform effectively. Second, this solution cannot correct for short-term temporal variations in pixel offset error that occur during operation of the array. For instance, variations in temperature of the detector array can create significant offset variations over time. Finally, this method requires recalibration to correct for long-term unpredictable changes in pixel offset errors that occur as the array components age.
An alternative approach eliminates the disadvantages associated with factory calibration by calibrating the focal plane array while it is in use. This is done by placing a rotating plate in front of the detector array, such that the array is alternately exposed to the image under observation and to a signal of known intensity. The fixed pattern noise is removed by subtracting a detector's response to the known signal from the detector's response to the observed image.
This solution has two drawbacks. First, by requiring a means for alternately exposing the array to the observed image and to a signal of known intensity, this solution requires additional complex mechanical or optical elements. Second, by requiring that the focal plane array spend time viewing a signal of known intensity instead of the scene under observation, this solution inevitably degrades the array's ability to track fast moving objects and reduces the potential signal to noise ratio of the sensor output. This approach is also not feasible when using TDI because you cannot stop to capture dark information without losing information about the scene.
O'Neil, in U.S. Pat. No. 5,514,865 which is incorporated herein by reference, discloses another approach for correcting spatial non-uniformities in a detector array. The O'Neil system employs a dithering system that spatially dithers the observed image across the detector array to correct the gain and offset errors in the array of detectors. The detector array line of sight is moved between consecutive image frames according to a predetermined pattern. This dithering of the array's line of sight causes different detectors to image the same location in the scene during different image frames, and causes two adjacent detectors to scan between the same two points in the scene during a cycle of the predetermined dither pattern. Theoretically, if two ideal detectors view the same part of an image then the two ideal detectors generate the same response to that part of the image. Differences existing in the response of two detectors viewing the same part of an image can accordingly be characterized as error in the detector response.
Dong, in U.S. Pat. No. 6,914,627 which is incorporated herein by reference, discloses another approach for correcting FPN noise. The Dong system uses a reference row of the pixel arrays that is covered by a light shield to collect a fixed pattern noise signal that can then be subtracted from the signals from the pixel array to cancel the fixed pattern noise.
A need therefore exists for improved fixed-pattern noise-reduction in a CMOS TDI image sensor. Accordingly, a solution that addresses, at least in part, the above and other shortcomings is desired.